As an energy storage device such as a lithium ion secondary battery, there has been known an energy storage device which includes an electrode assembly formed by overlapping a positive electrode plate, a negative electrode plate, and a sheet-like separator having an electric insulation property. For example, JP 2004-273178 A (patent literature 1) describes an energy storage device which includes an electrode assembly formed of a winding electrode. The winding electrode is formed by stacking and winding a set formed of an elongated insulation sheet, an elongated positive electrode sheet, an elongated insulation sheet and an elongated negative electrode sheet. In the winding electrode, a positive electrode foil of the positive electrode sheet, which projects from one long side of the insulation sheet, is stacked, and a negative electrode foil of the negative electrode sheet, which projects from the other long side of the insulation sheet, is stacked. The stacked positive electrode foil is joined to plate portions of a positive electrode terminal which sandwich the stacked positive electrode foil from both sides. The stacked negative electrode foil is joined to plate portions of a negative electrode terminal which sandwich the stacked negative electrode foil from both sides. In performing the joining, two plate portions which sandwich the electrode foil therebetween are plastically deformed such that one plate portion and the positive electrode foil or the negative electrode foil are sunk into the other plate portion.